Ab initio and density functional studies on the structure and vibrational spectra of 2-hydroxy-1,4-naphthoquinone-1-oxime derivatives

Structure and vibrational frequencies of lawsoneoxime and its C₃-substituted (R=CH₃, NH₂, Cl, NO₂) derivatives in keto and nitrosophenol forms have been obtained employing the Hartree–Fock and density functional methods. Charge distributions in different conformers have been studied using the molecular electrostatic potential topography as a tool. For all these derivatives except for nitrolawsoneoxime the amphi conformer in the keto form is predicted to be of lowest energy, which can partly be attributed to hydrogen bonding through the oximino nitrogen. In the nitro derivative, however, the preference to form a six membered ring owing to O–H–O hydrogen-bonded interactions makes the anti conformer (keto) the stablest. Further one of the nitrosophenol conformers of nitrolawsoneoxime turns out to be very close in energy (0.21 kJ mol^–1 higher) to this anti conformer. The consequences of hydrogen bonding on charge distribution and vibrational spectra are discussed.     

1H NMR spectra and ring shapes of the 4,5,8-trimethyl and 4-methyl derivatives of 1-tetralone

The 220 MHz spectra of the title compounds are reported and values for the internal ring dihedral angle, Ψ, formed between the C-2? C-1 and C-3? C-4 bonds, calculated using the R-value method. The trimethyltetralone exists in a conformer having Ψ = 51° and the 4-methyl group in the axial position. 4-Methyl-1-tetralone exists in a conformational equilibrium where the mean value of Ψ for the conformers adopted is 56°.     

Ab initio molecular orbital study of energies and conformers of 3,4-dihydro-1,2-dithiin, 3,6-dihydro-1,2-dithiin, 4H-1,3-dithiin,and 2,3-dihydro-1,4-dithiin

Optimized geometries and energies for 3,4-dihydro-1,2-dithiin ( 1 ), 3,6-dihydro-1,2-dithiin ( 2 ), 4H-1,3-dithiin ( 3 ), and 2,3-dihydro-1,4-dithiin ( 4 ) were calculated using ab initio 6-31G* and MP2/6-31G*//6-31G* methods. At the MP2/6-31G*//6-31G* level, the half-chair conformer of 4 is more stable than those of 1 , 2 , and 3 by 2.5, 3.5, and 3.6 kcal/mol, respectively. The half-chair conformers of 1 , 2 , 3 , and 4 are 2.9, 7.1, 2.0, and 5.6 kcal/mol, respectively, more stable than their boat conformers. The calculated half-chair structures of 1 – 4 are compared with the calculated chair conformer of cyclohexane and the half-chair structures for cyclohexene, 3,4-dihydro-1,2-dioxin ( 5 ), 3,6-dihydro-1,2-dioxin ( 6 ), 4H-1,3-dioxin ( 7 ), and 2,3-dihydro-1,4-dioxin ( 8 ). © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1064–1071, 1998     

Circular dichroism,proton magnetic resonance and conformation of steroid 4-en-3-ones, 4, 9-dien-3-ones and 4, 9, 11-trien-3-ones

Room and low-temperature CD of the title dienones and trienones (3 to 9) clearly show that a conformational equilibrium occurs between two ring A half-chair conformers. The relative stability of the two conformers depends on substitution of ring A. These results may be extended to other series of steroids and in particular to 4-en-3-ones. CD of these enones (14 to 28) is consistent with a conformational equilibrium between the known quasi-cis-quasi-trans conformers. The controversial conformational behaviour of 2 β-substituted 19-nor-4-en-3-ones is explained by a dynamic equilibrium in solution between the two afore-mentioned conformers rather than by single twist or deformed boat conformer.     

Conformational preferences of fluorocyclohexane and 1-fluoro-1-silacyclohexane molecules: ab initio study and NBO analysis

The molecular structure of the axial and equatorial conformers of fluorocyclohexane and 1-fluoro-1-silacyclohexane, H₂C(CH₂CH₂)₂XH–F (X = C or Si), as well as the thermodynamic equilibrium between these species were investigated by means of quantum chemical calculations up to MP2/aug-cc-pVTZ level of theory. According to MP2 data, these compounds consist of a mixture of conformers with chair conformation and C _s symmetry differing in the axial and equatorial position of the C–F/Si–F bonds (axial = 42/56 mol%, equatorial = 58/44 mol%) at T = 298 K. This corresponds to a free energy difference of A = (G _ax ? G _eq) = 0.19/?0.13 kcal mol^?1 for X = C/Si. NBO analysis revealed that the axial conformer of 1-fluoro-1-silacyclohexane is an example of stabilization of the form that is unfavorable in terms of conjugation effects; stabilization is achieved mainly due to steric interactions. The equatorial conformer of fluorocyclohexane is an example of stabilization of the form that is unfavorable in terms of electrostatic interactions; stabilization is achieved due to steric and conjugation effects.     

Photoisomerization of cis‐1,2‐di(1‐Methyl‐2‐naphthyl)ethene at 77 K in Glassy Media

cis‐1,2‐Di(1‐methyl‐2‐naphthyl)ethene, c‐ 1,1 , undergoes photoisomerization in methylcyclohexane, isopentane and diethyl ether/isopentane/ethanol glasses at 77 K. On 313 nm excitation the fluorescence of c‐ 1,1 is replaced by fluorescence from t‐ 1,1 . Singular value decomposition reveals that the spectral matrices behave as two component systems suggesting conversion of a stable c‐ 1,1 conformer to a stable t‐ 1,1 conformer. However, the fluorescence spectra are λ_exc dependent. Analysis of global spectral matrices shows that c‐ 1,1 is a mixture of two conformers, each of which gives one of four known t‐ 1,1 conformers. The λ_exc dependence of the c‐ 1,1 fluorescence spectrum is barely discernible. Structure assignments to the resolved fluorescence spectra are based on the principle of least motion and on calculated geometries, energy differences and spectra of the conformers. The relative shift of the c‐ 1,1 conformer spectra is consistent with the shift of the calculated absorption spectra. The calculated structure of the most stable conformer of c‐ 1,1 agrees well with the X‐ray crystal structure. Due to large deviations of the naphthyl groups from the ethenic plane in the conformers of both c‐ and t‐ 1,1 isomers, minimal motion of these bulky substituents accomplishes cis → trans interconversion by rotation about the central bond.     

Conformational analysis of 1,2,3,4-tetrahydroisoquinolines

Ring and nitrogen inversion account for the conformational equilibria of 3-phenyl-1, 2,3, 4-tetrahydroiso-quinolines. In order to quantitate the relative contribution of each conformer to the equilibrium, we undertook a molecular mechanics study on several substituted 3-phenyl-1, 2, 3, 4-tetrahydroisoquinolines. Predictions from calculations were checked against cmr chemical shift data. No boat conformation contributed significantly to the equilibrium. A general result of our calculations is that in all cases the 3-phenyl group in the equatorial position is strongly favored (by at least 2.50 kcal/mole). For 3-phenyl-1, 2, 3, 4-tetrahydroisoquinolines without substitution at nitrogen, N-H in equatorial position is preferred over the axial conformer, although the energy difference between both is always small (0.30–1.10 kcal/mole). For the cis-1,3-disubstituted compounds the le'3e conformers are the only species present (at least 99.8%). The calculated energy differences between the la′3a conformer and the le′3e conformer are always large (3.80–6.10 kcal/mole for the NHe conformers and 3.60–3.80 kcal/mole for the NHa conformers). The lack of a γ_1a upfield shift at C3 also points to the preference for the pseudoequatorial-equatorial conformer. For N-methyl-3-phenyl-1,2,3,4-tetrahydroisoquinoline a preference for the NMe group in the equatorial position is predicted (0.60–2.00 kcal/mole). The small downfield shift at C4 (γ_Na = 0.5 ppm) is consistent with the equatorial NMe preference. For the cis-1,2,3-trisubstituted compounds no significant γ_1a effect at C3 (γ_1a = -0.2 and 1.0 ppm) or γ_Na effect at C4 (γ_Na = 0.1 and 0.4 ppm) is observed. For these compounds, deformations due to steric congestion are evidenced by the deviation from the values of the C4a-C8a-Cl-N and C4a-C4-C3-N torsional angles, as compared to less crowded 3-phenyl-1,2,3,4-tetrahydroisoquinolines. Here the heterocyclic ring adopts a distorted half-chair conformation.     

Conformational Composition of Stereoisomers of 2-Alkyl-5-isopropyl-4-methyl-1,3,2-dioxaborinanes

¹H and ¹³C NMR spectroscopy was used to assign the configuration of a mixture of stereoisomers of 2-alkyl-5-isopropyl-4-methyl-1,3,2-dioxaborinanes with different ratios of the cis and trans forms. These forms differ in the configuration of C⁽⁴⁾ in the ring. The results of MM+ and AM1 calculations for the optimal geometry indicate high conformational flexibility of both isomers, which exist in an equilibrium mixture of sofa and half-chair conformers.     

Influence of structural features of isomeric hydrocarbons on ion formation at atmospheric pressure

Ion mobility spectrometry (IMS) in combination with different techniques of atmospheric pressure ionization (⁶³Ni ionization, photoionization, Corona discharge ionization) was applied to determine the influence of structural features of aromatic and cyclic hydrocarbons on ion mobility spectra. For this purpose, different sets of isomeric hydrocarbons were investigated using the above-mentioned ionization techniques. We found different structural features of these isomeric non-polar compounds which cause distinct differences in ion mobility spectra. These differences result from the formation of different product ions or a different relative abundance of ions formed depending on the occurrence of certain structural features (position of the double bond, arrangement of double bonds within the carbon ring, configuration of aliphatic side chain in the space, position of aliphatic side chain on the carbon ring and the number of carbon atoms in the aliphatic side chain). The nature of product ions formed was determined using a coupling of IMS with mass spectrometry (MS).     

Les dimethyl-2ξ, 17α,δ9 et δ9,11 nor-19-testosterones. Conformation du cycle a et equilibres en solution

The preparation of 2α- and 2β-isomers of the 2,17-dimethyl-17β-hydroxy estra-4,9 diene-3-ones, 4 and the 2,17-dimethyl-17β-hydroxy estra-4,9,11 triene-3-ones, 21, is described and the configuration at C₂ established by chemical means. CD spectra show that these compounds exist in solution as an equilibrium mixture of two conformers, the proportion depending on the configuration of the methyl group on carbon 2. It appears that ring A of the 2β isomers is in a preferred “inverted half-chair” conformation while the “normal half-chair” conformation is favoured with the methyl group in the 2α-configuration. In alkaline medium the methyl group at C₂ of the dienones 4 and the trienones 21 is partially isomerised and at equilibrium the resulting CD spectra are identical to those of the unsubstituted dienone 1 and trienone 15. An explanation of this phenomenon is given.     

Alignment and ordering mechanisms at a liquid crystal–solid interface

Abstract

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal–solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0·2 μm in diameter are chemically modified using an aliphatic acid (C _n H_2n+1 COOH) as a surface coupling agent and filled with the liquid crystal compound 4′-pentyl-4-cyanobiphenyl deuteriated in the α position of the hydrocarbon chain (5CB-αd ₂). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n ≥7 while chain lengths n ≤6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n = 15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.     

Microwave, Raman and infrared spectra, r0 structural parameters, conformational stability, and ab initio calculations of cyclobutylisocyanate

The microwave spectrum of cyclobutylisocyanate, c-C₄H₇NCO, has been investigated from 21,000 to 11,000 MHz and 11 transitions for the more stable equatorial-trans conformer were assigned. The rotational constants of the ground vibrational state have been determined and the molecule has been shown to be a near symmetric prolate rotor (К = ?0.99). The B and C rotational constants have been confidently determined to be B = 1508.68(3) and C = 1476.55(2) MHz, respectively, whereas the value for the A rotational constant of 6,891(302) MHz had a large uncertainty. Variable temperature (?100 to ?55 °C) studies of the infrared spectra (3,500–400 cm^?1) of cyclobutylisocyanate dissolved in liquid xenon as well as the infrared spectra of the gas and solid have been recorded. In addition, the Raman spectra (3,600–100 cm^?1) of the liquid have been investigated. These spectral data indicated the present of three conformers in the fluid states which are the equatorial-trans, equatorial-gauche, and axial-trans forms. The second part of the conformational name refers to the relative position of the NCO moiety relative to the alpha hydrogen. By utilizing four conformer pairs, an enthalpy difference of 131 ± 13 cm^?1 (1.57 ± 0.16 kJ/mol) was obtained with the equatorial-trans conformer the more stable form, which is in good agreement with the ab initio predicted value of 137 ± 36 cm^?1 (1.64 ± 0.43 kJ/mol). To aid in the vibrational assignment, ab initio and DFT calculations have been carried out by using a variety of basis sets up to 6-311G(3df,3pd).     

Vibrational-torsional excitation and direct overtone photodissociation of ethyl hydroperoxide at 5nuOH

We have observed OH radical products from the unimolecular dissociation of ethyl hydroperoxide (CH3-CH2OOH) excited to 5nuOH and have collected an action spectrum from 15,600 to 16,800 cm(-1) and an OH product state distribution at the maximum (16,119 cm(-1)). We use a vibrational-torsional model to simulate spectra in the 5nuOH region for the trans and gauche conformers. A combination of the two simulated spectra resembles the experimental action spectrum, provided that the trans conformer is assumed to dominate at room temperature. Energy disposal in the OH fragment yields an upper limit for the O-O bond dissociation energy at D0 < 44 kcal mol(-1).     

Conformation of Some Anomeric O-Benzylated D-gluco-, L-Ido-, and Hex-4-enopyranoside derivatives

By use of ¹H-NMR. spectroscopy it is shown that in the anomeric tri-O-benzyl derivatives of D -gluco- and L -idopyranosides the C(5) (aliphatic) substituent has a strong tendency to assume an equatorial position. The conformer (⁴C₁ or ¹C₄) with C(6) in equatorial position is favoured, even if other bulky substituents are forced to occupy an axial position; the same effect is observed in some anomeric O-benzylated 6-deoxy-L -idopyranoside derivatives. If, however, the tetragonal configuration of the saturated C(5) is changed into a trigonal (sp²) configuration, e.g. in the anomers of 1,2,3-tri-O-benzylated L -threo-hex-4-enopyranosiduronates, the C(5)–C(6) bond no longer determines the equilibrium of (distorted) pyranoid conformations; the anomeric (‘Eduard-Lemieux’) effect then becomes important in the stabilization of the corresponding half-chair (¹H₂ or ²H₁) conformations.     

Mixed Boron(III) and Phosphorous(V) Complexes of meso‐Triaryl 25‐Oxasmaragdyrins

Two unprecedented mixed B^III/P^V complexes of meso‐triaryl 25‐oxasmaragdyrins were synthesized in appreciable yields under mild reaction conditions. These unusual 25‐oxasmaragdyrin complexes containing one or two seven‐membered heterocyclic rings comprised of five different atoms (B, C, N, O, and P) were prepared by reacting B(OH)(Ph)‐smaragdyrin and B(OH)₂‐smaragdyrin complexes, respectively, with POCl₃ in toluene at reflux temperature. The products were characterized by HRMS and 1D‐ and 2D‐NMR spectroscopy. X‐ray crystallography of one of the mixed B^III/P^V smaragdyrin complexes indicated that the macrocycle is significantly distorted and contains a stable seven‐membered heterocyclic ring within the macrocycle. The bands in the absorption and emission spectra were bathochromically shifted with reduced quantum yields and singlet‐state lifetimes relative to the free base, meso‐triaryl 25‐oxasmaragdyrin. The mixed B^III/P^V complexes were difficult to oxidize but easier to reduce than the free base. The DFT‐optimized structure of the 25‐oxasmaragdyrin complex with two seven‐membered heterocycles indicated that it was a bicyclic spiro compound with two half‐chair‐like conformers. This was in contrast to the chair‐like conformation of the complex with a single seven‐membered heterocyclic ring. Moreover, incorporation of a second phosphate group in the former case stabilized the bonding geometry and resulted in higher stability, which was reflected in the bathochromic shift of the absorption spectra, more‐positive oxidation potential, and less‐negative reduction potential.     

Conformational Stability from Variable-Temperature Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r o Structural Parameters of Chlorocyclopentane

The infrared spectra (3500–400 cm^–1) of krypton solutions of chlorocyclopentane, c-C₅H₉Cl, at variable temperatures (–101 to –150°C) have been recorded and the fundamental vibrations of the axial conformer and several of those for the equatorial form have been assigned. Utilizing two pairs of fundamentals for the two conformers in the krypton solution, an enthalpy difference of 145±15 cm^–1 (1.73±0.18 kJ-mol^–1) has been obtained with the axial conformer the more stable form. It is estimated that there is 67±2% of the axial conformer present at ambient temperature. Convincing spectroscopic evidence shows that a significant percentage of the chlorocyclopentane molecules are undergoing pseudorotation at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation and these quantities have been compared to the experimental values when appropriate. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2 calculations as well as by density functional theory (DFT) by the B3LYP method with several different basis sets. The adjusted r ₀ structural parameters have been obtained for both conformers by combining the ab initio data with the previously reported microwave rotational constants. These new values of the structural parameters for both conformers are compared to those previously reported from electron diffraction and microwave studies. These results are compared to the corresponding quantities of some similar molecules.     

Mass spectometry of aralkyl compounds with a functional group—VI1: Mass spectra of 1-phenylethyanol-1, 2-phenylethanol-1 and 1-phenylpropanol-2

Mass spectra of 1-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain, suggest that the [M? CH₃]⁺ ion is represented partly by an α-hydroxybenzyl fragment. Moreover, the molecular ion loses successively—after scrambling of all hydrogen atoms, except those of CH₃? a hydrogen atom and C₆H₆, generation the CH₃CO⁺ ion. Diffuse peaks, found in the spectra of of 2-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain and in the phenyl ring, show that the molecular ion loses C₂H₄O, possibly via a four-center mechanism, after an exchange of aromatic and hydroxylic hydrogens. Mass spectra of 1-phenylpropanol-2 and its analogues, specifically, deuterated in the aliphatic chain, demonstrate that in the molecular ion exclusively the hydroxyl hydrogen atom is transferred to one of the ortho-positions of the phenyl ring via a McLafferty rearrangement, generating the [M ? C₂H₄O]⁺ ion. Furtherore, an eight-membered ring structure is proposed for the [M ? CH₃]⁺ ion to explain the loss of H₂O and C₂H₂O from this ion after an extensive scrambling of hydrogen atoms.     

Conformational isomerism of 1-butene secondary ozonide as studied by means of matrix isolation infrared absorption spectroscopy

Theoretical calculations of structures, stability and vibrational spectra of 1-butene secondary ozonide (SOZ) conformers were performed using DFT method B3LYP with a 6-311++G(3df, 3pd) basis set. The calculations predict six staggered structures of 1-butene SOZ. The FTIR spectra of 1-butene SOZ isolated in Ar, N₂ and Xe matrices were recorded. It was found that nitrogen is the best suited for the matrix isolation of 1-butene SOZ. The bandwidth of the spectral bands of the ozonide isolated in nitrogen was as narrow as 2 cm⁻¹. For the first time the existence of five conformers of 1-butene SOZ were confirmed experimentally by means of matrix isolation infrared absorption spectroscopy. The equatorial gauche (∠OCCC=−66.1°) conformer was proved theoretically and experimentally to be the most stable. It was found that due to high potential barriers of the conformational transitions annealing of the matrix is useless for the assignment of spectral bands to various conformers of 1-butene SOZ. Using the hot nozzle technique the van’t Hoff experimental plots were made for three additional conformers of 1-butene SOZ and experimental ΔH values for these additional conformers were established. The crystallization problems of 1-butene SOZ are discussed which accounts for the rich conformational diversity of the ozonide as well as high conformational barriers for axial-equatorial transitions.      

Conformational Studies of Marine Polyhalogenated α-Chamigrenes Using Temperature-Dependent NMR Spectra. Cyclohexene-ring flipping and rigid-chair cyclohexane ring in the presence of equatorial halogen atoms at C(8) and C(9)

Temperature-dependent NMR spectra indicate that the α-chamigren-3-ones (?) -11 , (+) -12 , (+) -14 (?) -15 , (+) -16, 18 , and 19 bearing equatorial halogen atoms at C(8) and C(9) undergo slow conformational flipping of the envelope-shaped enone ring, while the cyclohexane ring is maintained in the chair conformation. The α-chamigren-3-ols (+) -20 and (+) -21 , obtained by hydride reduction of (+) -12 , behave similarly, with slow half-chair inversion of the cyclohexenol ring. In each case, both conformers are about equally populated and detectable by NMR, except in the case of (+) -15 , where repulsive interactions between Br? C(2) and H_eq?C(7) make the population of the conformer 15b with Me—C(5) faced to H_ax?C(10) so low that it escapes direct ¹H-NMR detection. The energy barriers to these conformational motions are viewed to arise mainly from repulsive interactions between Me—C(5) and the axial H-atoms at C(8) and C(10), while, contrary to previous beliefs, no twist-boat conformations of the cyclohexane ring intervene. Similar conclusions hold for the 4,5-epoxides of both (?) -6 and (+) -7 . Clean Jones oxidatio of (?) -2 to 17 , where the CH₂?C(5) bond is maintained, and acid dehydration-isomerization of the α-chamigrene (+) -21 to the β-chamigrene (+) -24 , reflect the special stability of β-chamigrenes, providing a reason for their frequent occurrence in nature.     

Conformational analysis of vitamin K<Subscript>1</Subscript> model molecule: a theoretical study

Isomerism, conformations, and molecular structure of a model molecule of vitamin K₁ with a truncated side chain have been studied by the density functional theory calculations using B3LYP method and double- and triple-ζ correlation consistent basis sets. The conformations of two possible (E and Z) isomers, formed by the rotations around three single C–C bonds closest to the naphthoquinone ring, have been studied. The lowest energy conformers are stabilized by additional hydrogen bonds between hydrogen atoms of the side chain and an oxygen atom in the naphthoquinone subunit. It is interesting to note that the structure of the energetically preferred conformer of the E-isomer (3c) has been found to be similar to the solid state structures of phylloquinones in the photosystem I of cyanobacterium Synechococcus elongatus. The excited electronic states of two lowest energy conformers have also been investigated.